1. Field of the Invention
The present invention relates generally to an electric power supply employing a switching circuit to convert an ac voltage to a stabilized dc voltage. More specifically, the invention relates to an electric power supply capable of improving a power factor thereof.
2. Description of the Related Art
A known circuitry configuration for an electric power supply to improve a power factor thereof is shown in FIG. 5. In the circuit of FIG. 5, an output of a rectifying circuit 10 for full-wave rectifying an ac input voltage in, is input to a boost chopper circuit. The chopper circuit outputs a stabilized dc voltage.
The chopper circuit in FIG. 5 comprises a switching device Q1 driven with sufficiently higher frequency than that of the input ac voltage, an inductor L1 connected in series with the switching device Q1 between output terminals of the rectifying circuit 10, and a diode D1 and a capacitor C1 connected in series between the input and output terminal of the switching device Q1 in order to let an output current flow through the inductor L1 during the OFF state of the switching device Q1. The capacitor C1 has a substantially large capacity so that a smoothed and stabilized dc output voltage is supplied therefrom. It should be noted that a capacitor C2 in the input side of the chopper circuit is disposed for absorbing a high frequency ripple, with a small capacity, and is dispensable for the circuit.
An output voltage V2 of the chopper circuit is controlled so as to comply with a reference voltage Vs. An error voltage between V2 and Vs is detected by an error amplifier 11, and the output signal of the amplifier 11, an error signal .DELTA.V, is input to a multiplier 12. An input voltage V1 (i.e., a full-wave rectified input ac voltage) is also input to the multiplier 12. The multiplier 12 multiplies V1 of the chopper circuit by .DELTA.V to generate and output a threshold signal S0 with a full-wave rectified waveform, in which its phase is same as that of the input voltage of the chopper circuit, and its amplitude corresponding to the signal .DELTA.V.
The instantaneous value of the current through the switching device Q1 in the chopper circuit is detected by a resistor R1. A current detection signal S1 measured as a voltage drop of R1 is compared with the threshold signal S0 by a comparator 13. During the ON state of Q1, the current flowing through Q1 via the inductor L1 keeps increasing gradually. The current detection signal S1 also increases along with the Q1 current increase and the output signal of the comparator 13 turns HIGH when S1 reaches S0. The output signal of the comparator 13 resets a flip-flop 14.
The circuit shown in FIG. 5 is so constructed that a driver 15 makes the switching device Q1 turn on while the Q output of the flip-flop 14 is maintained HIGH; therefore, the flip-flop 14 is reset when the current detection signal S1 representing the current through the switching device Q1 reaches the threshold signal S0. Consequently, the Q output of the flip-flop 14 turns LOW and the switching device Q1 is forced to turn off.
As shown in FIG. 6, the current flowing to the output side from the inductor L1 via the diode D1 gradually decreases after the switching device Q1 turns off. The inductor L1 is provided with a secondary winding Ws for detecting the current through the primary winding of L1. When Q1 turns off, the back electromotive force of L1 sets the flip-flop 14. The output signal of the driver 15 is set HIGH via an AND circuit 16 at the time the current of L1 falls to zero.
Namely, when the current through the inductor L1 decreases to be zero, the switching device Q1 turns on and the current through the inductor L1 and the switching device Q1 gradually increases. Then, when the current through L1 and Q1 reaches to the threshold signal S0 value, Q1 turns off and the current through L1 gradually decreases. According to the above described repeating operations, as shown in FIG. 6, the switching device Q1 is driven to ON or OFF with sufficiently higher frequency than that of the ac input voltage and the dc output voltage is so controlled that the envelope of the current through the inductor L1 follows the threshold signal S0, i.e., a full-wave rectified waveform of the ac input voltage.
As is apparent from the above description, in the existing electric power supply shown in FIG. 5, the waveform of the instantaneous current through the inductor L1 consists of a series of serrated pulses and the envelope thereof forms a full-wave rectified waveform as shown in FIG. 6. Therefore, as the peak value of the output current becomes substantially larger than the effective value thereof, a large ripple current flows into a capacitor connected to the input line (not shown) and the capacitor C2 with a relatively small capacity for absorbing ripple component of the input voltage. As a result, those capacitors become heated and/or line reflection noise may increase. Moreover, when this circuit configuration is applied to a large capacity electric power supply, the switching device Q1 is required to have a large rated current in order to endure the peak value of the serrated current larger than the effective current. Additionally, the inductor L1 included in the above existing electric power supply is provided with the secondary winding Ws and thus, the inductor L1 becomes expensive.
The circuit arrangement of an electric power supply designed for overcoming the above problems of the circuit in FIG. 5 is shown in FIG. 7. This circuit is so constructed that an oscillator 17 is connected to the set (S) terminal of the flip-flop 14 and the output signal of the comparator 13 is input to the reset (R) terminal of the flip-flop 14.
The operation of the circuit in FIG. 7 will be described herebelow. The instantaneous value of the current passing the switching device Q1 in the chopper circuit is detected as a terminal voltage of the resistor R1. The terminal voltage of R1 is input to the non-inverting terminal of the comparator 13 as the current detection signal S1. The comparator 13 compares S1 with S0 input to the inverting terminal of the comparator 13 from the multiplier 12.
The flip-flop 14 and the driver 15 driving the switching device Q1 force the device Q1 to turn on in response to the pulse signal input from the oscillator 17, with sufficiently higher frequency than that of the ac input voltage, and make Q1 turn off for the output of the comparator 13. During ON state of Q1, the current passing the device Q1 via the inductor L1 gradually increases. When the current detection signal S1 reaches the threshold signal S0, the Q output of the comparator 13 turns to HIGH. The signal appearing at the Q output of the comparator 13 resets the flip-flop 14 to force the device Q1 to turn off. As the above circuit operations are repeated in synchronism with the high frequency pulses input from the oscillator 17, the dc output voltage is so controlled that the envelope of the current through the inductor L1 complies with the threshold signal S0, i.e., a full-wave rectified waveform of the ac input voltage.
According to the circuit construction in FIG. 7, as shown in FIG. 8(a), the peak value of the current in the inductor L1 can be reduced compared with that of the circuit in FIG. 5, thus the heating of the capacitors and the increase in the line reflection noise are relieved. However, as the device Q1 is driven by constant frequency pulses and the acute peak of the line reflection noise appears around the oscillation frequency of the oscillator 17, a noise filter with large damping effect must be employed in the circuit. Moreover, the difference between the input voltage and the output voltage is so large that the current through the inductor L1 cannot reach the threshold signal S0 and the input current waveform cannot become a sine wave because of inevitable distortion even though the ON pulse width of the driving pulses is maximized. (FIGS. 8 (a),(b))